Lt. Gen. James Peake, the Army's surgeon general, has sent two doctors and four other experts to Iraq and two more doctors to Landstuhl Regional Medical Center in Germany, where some of the troops were treated after being flown from Iraq, officials said.

"It is pneumonia. The question is what is the cause," said Lyn Kukral, spokeswoman for Peake and the Army Medical Command.

"The epidemiological teams will look and follow the facts wherever they lead," Kukral added. "You've got a healthy population and a young population (US troops), and you have two soldiers who have died, and that's a concern."

Kukral said there have been more than 100 cases among US troops in the Iraq region since the beginning of March, including 15 serious enough to warrant medical evacuation to get the patients ventilators to assist their breathing.